Potentiometers



March 10, W. o. KIRKENDALL 3,

POTENTIOMETERS Filed Feb. 5, 1968 3 Sheets-Sheet 5 J v ///(//2) INVENTOR WILL/AM D. K/RKENDHLL Fi .1]. I

wwzm United States Patent 3,500,281 POTEN TIOMETERS William D. Kirkendall, Dalton, Pa., assignor to Weston Instruments, Inc., Newark, N.J., a corporation of Delaware Filed Feb. 5, 1968, Ser. No. 702,950 Int. Cl. H01c /02 US. Cl. 338-180 16 Claims ABSTRACT OF THE DISCLOSURE Rectilinearly adjusted, lead screw-operated potentiometers, particularly of the miniaturized type, are provided having an improved movable contact unit including a thin-walled tubular carriage member of flexible insulating material, and a spring metal movable Contact member having a portion which embraces the tubular carriage member and includes a protuberance which deforms a limited area of the thin wall of the carriage member inwardly into engagement between adjacent turns of the thread of the lead screw, the balance of the tubular wall of the carriage member being supported, but not significantly distorted, by the crest of the thread of the lead screw.

This invention relates to potentiometers of the type employing an elongated resistance element, a movable contact arranged to be adjusted rectilinearly relative to the resistance element, and a lead screw for driving the movable contact, and is particularly directed to improved movable contact units for such otentiometers. The invention is an improvement on potentiometer structures of the type disclosed in my US. Patent 3,358,259, issued Dec. 12, 1967, and is particularly applicable to miniaturized otentiometers.

In rectilinearly adjusted potentiometers, particularly when miniaturized, it has been difficult to provide a movable contact and lead screw structure which is fully satisfactory from both the mechanical and electrical standpoint. Difiiculties have been accentuated when a metal lead screw is employed, since it is then necessary to insulate the movable contact unit electrically from the lead screw, while achieving a dependable screwthread engagement and also providing a clutch action effective to prevent damage when turning of the lead screw is continued after the movable contact unit has come into engagement with one of the end stops.

It is accordingly a general object of the invention to provide, in a potentiometer of the type described, an improved movable contact unit having a particularly simple and effective means for achieving a driving interengagement between the contact unit and the lead screw.

Another object is to devise a movable contact structure having improved and simplified means for insulating the contact structure electrically from the lead screw when a metal screw is employed.

A further object is to achieve improved positional stability of the movable contact at all points in its travel along the lead screw.

Polentiometers according to the invention can employ a conventional metal lead screw, and the improved contact unit comprises a thin tubular insulating member which lightly embraces the crest of the lead screw thread, advantageously in such fashion that, though substantially free relative movement between the screw and insulating member is allowed, the tubular body of the insulating member is positively supported by the lead screw against overall inward deformation. The unit also comprises an integral contact member formed of spring sheet metal and including a body portion and at least "ice one contact arm. The body portion of the contact member has a protuberance which is significantly smaller than the space between adjacent turns of the lead screw thread and which is so disposed as to be urged against the thin Wall of the tubular body of the insulating member by a spring force obtained by deformation of the body portion of the contact member, such deformation resulting from the manner in which the body portion of the contact member is engaged with the insulating member. The protuberance is thus caused to deform a small area of the thin wall of the insulating member inwardly into the space between the adjacent turns of the lead screw thread. The protuberance on the body portion of the contact member thus acts as a nut member, and the insulating material deformed thereby is simply provided to prevent electrical contact between the contact member and the lead screw and to afford some antifriction effect. Advantageously, the lead screw thread has a rounded crest to afford simple support for the undeformed portion of the wall of the insulating member. Alternatively, the lead screw can be provided with a thread having a sharp crest, and the protuberance on the contact member can be enlarged to force a larger area of the thin wall of the insulating member inwardly against portions of two or more turns of the thread, so that the crest of the thread is actually forced into screw thread engagement with the portion of the insulating member deformed by the protuberance.

In order that the manner in Which the foregoing and other objects are attained in accordance with the invention can be understood in more detail, one particularly advantageous embodiment thereof will be described with reference to the accompanying drawings, which form a part of this specification, and wherein:

FIG. 1 is a longitudinal sectional view of a potentiometer constructed in accordance with the invention;

FIG. 2 is a transverse sectional View taken on line 2-2, FIG. 1;

FIG. 3 is an enlarged longitudinal sectional view of the movable contact unit of the potentiometer of FIG. 1, with a portion of the lead screw shown in side elevation;

FIG. 4 is an end elevational view of the movable contact member in relaxed, undistorted condition;

FIG. 5 is a transverse cross-sectional view taken on line 5-5, FIG. 3;

FIG. 6 is a view similar to FIG. 5 of a movable contact unit according to another embodiment of the invention;

FIG. 7 is a longitudinal sectional view of a potentiometer constructed in accordance with another embodiment of the invention;

FIG. 8 is a transverse sectional view of the potentiometer of FIG. 7;

FIG. 9 is a side elevational view of the movable contact unit and a portion of the lead screw of the device of FIG. 7;

FIG. 10 is a perspective view of a subassembly employed in the potentiometer of FIG. 7 and comprising a return conductor and its lead element;

FIG. 11 is a top plan elevational view of the base of the potentiometer of FIG. 7, with the resistance element and return conductor mounted thereon;

FIG. 12 is a fragmentary view of a portion of a lead element installed in accordance with the invention, the lead element being in side elevation and the material of the casing broken away for clarity;

FIG. 13 is a fragmentary view similar to FIG. 12 but showing the lead element in edge elevation;

FIG. 14 is a side elevational view illustrating a modified mounting arrangement for the return conductor of a potentiometer otherwise constructed as shown in FIG. 7;

FIG. 15 is a fragmentary transverse sectional view of a potentiometer constructed as in FIG. 7 but with a modified movable contact unit;

FIG. 16 is a plan elevational view of the movable contact unit of FIG. 15;

FIG. 17 is a fragmentary side elevational view of the contact unit and lead screw of the device of FIG. 15; and

FIG. 18 is a view similar to FIG. 17 but showing the contact unit in modified form as applied to a different type of lead screw.

As seen in FIGS. 1 and 2, the potentiometer 1 comprises a casing 2 of rectangular configuration, including a top wall 3, bottom wall 4, side walls 5 and 6, and end portions 7 and 8. A metal lead screw 9 is journalled in end portions 7 and 8, has its slotted head 10 exposed beyond end portion 8, and includes a threaded body portion 11 which extends longitudinally of the casing for substantially the full distance between the inwardly directed transverse faces 12, 13 of the end portions 7 and 8, respectively. The lead screw thread has a rounded crest. Body portion 11 of the lead screw is adjacent top wall 3.

An elongated wire wound resistance element 14 extends longitudinally within the casing and engages both bottom wall 4 and side wall 5. A straight, elongated return conductor 15 extends longitudinally along the inner face of side wall 6, being accommodated by a suitable groove in the side wall. The threaded body portion 11 of the lead screw, the resistance element, and the return conductor are all mutually parallel.

The movable contact unit 16, FIGS. 3 and 5, includes a carriage member 17, which in this embodiment also constitutes an insulating member, and an integral spring sheet metal contact member 18. Insulating member 17 is an integral piece of a suitable flexible polymeric insulating material and comprises a thin tubular body 19 and spaced transverse annular outwardly projecting end flanges 20, 21 which are markedly thicker than body 19. When in relaxed, undistorted condition, body 19 extends as a right cylindrical tube having an inner diameter substantially equal to the diameter of the crest of the thread of lead screw body 11. The tubular body 19 of the in sulating member is slipped over the threaded body of the lead screw, so that the body 19 lightly embraces the crest of the thread of lead screw body. The length of the tubular body 19 is substantially greater than the pitch of the thread of the lead screw, its length advantageously being such that at least three fully turns of the thread are engaged by the inner surface of body 19, thus affording good positive support of body 19 against overall inward deformation.

Contact member 18 comprises a body portion 22 which extends cylindrically for slightly less than 360, so as to have edge portions 23, 24 from which the two contact arms and 26 respectively extend. When relaxed and undistorted, body portion 22 has a diameter somewhat smaller than the outer diameter of the tubular body 19 of insulating member 17, edge portions 23 and 24 are closely adjacent each other, and contact arms 25 and 26 are mutually parallel. The length of body portion 22 is slightly less than the space between end flanges 20, 21 of insulating member 17, so that body portion 22 can be snapped over the tubular body of the insulating member and, when so installed, will be distorted outwardly from its normal or relaxed condition by reason of the aforementioned diiference in diameters.

At a location midway between its ends and generally diametrically across from the two contact arms, body portion 22 of contact member 18 is provided with an inwardly projecting dimple or protuberance 27 of generally hemispherical configuration and having a dimension, in a direction axially of the assembly, significantly smaller than the pitch of the thread of the lead screw. Since the diflerence between the inner diameter of body portion 22 and the outer diameter of body 19 of the insulating member causes body portion 22 to be resiliently distorted outwardly, body portion 22 applies an inwardly directed spring force to body portion 19. Hence, where protuberance 27 engages body 19, a corresponding small portion of the thin wall of body 19 is deformed inwardly into the space between two adjacent turns of the lead screw thread.

Contact arm 25 terminates at its free end in a contact portion 28 whch slidably engages resistance element 14. Similarly, arm 26 terminates in a contact portion 29 which slidably engages the return conductor 15. When body portion 22 of the contact member embraces the lead screw, the resulting distortion thereof causes arms 25 and 26 to diverge outwardly to such an extent that the space between contact portion 28 and 29 is substantially greater than the lateral space between resistance element 14 and return conductor 15. Hence, in the completed assembly, the contact member is resiliently stressed in a sense tending to increase the spring force which acts to urge protuberance 27 inwardly, this stress resulting partly from bending of the junctures between arm 25 and edge portion 23, and between arm 26 and edge portion 24.

In effect, protuberance 27 forces a small portion of the wall of tubular body 19 into such engagement with the thread of the lead screw 9 that the combination of the insulating member 17 and contact member 18 acts as a nut, held against rotation by reason of engagement of contact portions 28 and 29 with resistance element 14 and return conductor 15, respectively, so that rotation of the lead screw causes the movable contact unit to travel along the lead screw body toward one or the other of the end portions 7, 8, depending upon. the direction in which the lead screw is rotated. Should such rotation bring the end face 30, presented by flange 20 of insulating member 17, into contact with the stop face 12 of casing end portion 7, travel of the movable contact unit is halted and continued rotation of the lead screw is still allowed because body 19 and the body portion 22 of the contact member yield resiliently to enable the portion of the wall which is deformed by protuberance 27 to ride over the crest of the thread of the lead screw. A similar clutch action occurs if the end face 31 presented by flange 21 comes into engagement with stop face 13a, presented by a flange at the end of lead screw body 11 adjacent face 13 of end portion 8, and rotation of the lead screw still continues.

Referring particularly to FIG. 3, it will be noted that the dimple 27 is of such size that all of that portion of the wall of body 19 which is deformed by the dimple lies between the adjacent turns of the lead screw thread. Thus, in effect, the dimple 27 does not deform the insulating material inwardly against the crest of the lead screw in such a fashion as would cause the crest to embed itself significantly into the polymeric insulating material of body 19.

In a typical embodiment of the invention as applied to miniaturized potentiometers, insulating member 17 is made as an integral piece of polytetrafluoroethylene, the length of the member being 0.120", the diameter of body 19 thereof being 0.070" and the thickness of the cylindrical tubular body 19 being 0.008. Polytetrafluoroethylene is a typical suitable material for member 17, combining good strength and electrical insulating properties with excellent flexibility and low friction characteristics.

While the tubular body 19 of the insulating member and the tubular body portion 22 of the contact member are of circular transverse cross section in the illustrative embodiment just described, these elements can be of other tubular configurations, without departing from the scope of the invention. Thus, for example, the tubular body of the insulating member can be of square transverse cross section, as seen at 19a, FIG. 6, with the body portion 22a of the contact member then extending in a like square configuration, and with the inner surface of body 19a slidably engaging the crest of the thread of the lead screw body 11a along four spaced lines.

In the embodiment of the invention shown in FIGS. 7-13, the housing 40 of the potentiometer comprises a base 41 of rectangular plan configuration, and a hollow cover or cap 42 secured to the base so that the base and cap define a generally rectangular cavity. Base 41 can be considered as providing one wall of the housing and this wall can be viewed as the bottom in the normal position of installation. Cap 42 includes a flat top wall 43, flat side walls 44 and 45, and end walls 46 and 47.

At its top, base 41 is provided with a continuous peripheral upstanding flange 48 having a narrow flat top surface and a downwardly and inwardly slanting inner face. The lower edges of side walls 44, 45 and end walls 46, 47 are peripherally notched to accommodate the flange 48. Base 41 and cap 42 are formed of a suitable thermoplastic polymeric insulating material and the two members are secured rigidly together, in sealed fashion, by ultrasonic welding in the manner hereinafter described. Thus, all of the interengaging surface portions of base 41 and cap 42 are fused together.

In that area enclosed by cap 42, base 41 is provided with a rectangular portion 49 which projects upwardly only slightly above the common plane of the fused junction between the base and the lower edge portions of the side and end walls. Portion 49 presents a flat upwardly facing surface 50. Adjacent side wall 44, an upstanding rim 51 extends parallel to the side wall. Rib 51 is of rectangular cross setcion and is interrupted by three notches 52-54 which extend downwardly completely to surface 50. A groove 55, of arcuate transverse cross section, extends lengthwise of rib 51 parallel to wall 44 and opening upwardly. The width of rib 51 is substantially smaller than the width of surface 50, so that a substantial portion of that surface is clear and uninterrupted, between the side surface 56 of rib 51 and the side wall 45 of cap 42. At the end of base 41 adjacent end wall 46, the base is provided with an upstanding pedestal '57 which joins the adjacent end of rib 51 and includes a flat surface 58 extending across the adjacent end of groove 55. Pedestal 57 projects well above rib 51, for a purpose hereinafter described.

Base 41 is provided with three lead-accommodating bores 59-61 which are identical and of rectangular transverse cross section. Bores 59-61 extend completely through the base at right angles to the plane thereof. Bore 59 is located in centered alignment with respect to notch 53 in rib 51 and is approximately centered below the inner face of side wall 44 of cap 42. At its upper end, bore 59 is enarged rectangularly, in a direction which is longitudinal with respect to the housing, the enlargement extending equally in both directions from the main portion of the bore and thus providing a rectangular recess 62 which is centered on the main portion of bore 59 and opens upwardly. Bores 60 and 61 are provided with recesses identical to recess 62.

Rib 51 supports a return conductor 63. Return conductor 63 is in form of a straight piece of wire of circular transverse cross section and of such diameter as to be accommodated by groove 55. The end of conductor 63 which is adjacent end wall 46 directly engages the face 58 of pedestal 57. The opposite end of the return conductor is spaced inwardly from end wall 47.

As best seen in FIG. 10, return conductur 63 is equipped with a lead element 64. Element 64 is made as an integral unit from suitable metal of good electrical conductivity and good strength. The element includes an elongated straight shank portion 65 of rectangular cross section, a relatively shorter cross-bar portion 66 which is also of rectangular cross section, and a thin tip portion 67. The central portion of cross-bar 66 lies in the same plane as shank 65, but the end portions 66a of the cross-bar are bent to project at right angles to that plane. both end portions projecting in the same direction. The cross-bar is of rectangular transverse cross section and of the same thickness as shank 65. One face of the central portion of cross bar 66 is coplanar with one major face 68 of shank 65. The tip 67 joins cross-bar 66 at the midpoint of the cross-bar but is oflset transversely so that one major face of the tip joins the cross-bar in a line which lies in the plane of major face 68. The tip 67 slants away from the lead element on the side thereof which is occupied by face 68. The free end of tip 67 is welded to the return conductur 63 at a point intermediate the ends of the return conductor. Lead element 64 is so oriented relative to the return conductor that the major surface of tip 67 which faces in the same direction as does face 68 engages the return conductor.

During assembly of the potentiometer, at a time before cap 42 has been applied to base 41, lead 64 is inserted downwardly into bore 59 until cross bar 66 seats in recess 62. The effective length and slanting disposition of tip 67 are such that, when cross bar 66 is first seated within recess 62, and before ultrasonic welding of the cap to the base has been accomplished, the return conductor 64 is approximately seated in groove 55, that is, the ends of the cross bar engage the flat bottom wall of the recess. Shank 65 extends with its major face 68 in flush engagement with the adjacent side of the main portion of bore 59. The length of shank 65 is such that, when cross bar 66 is seated in recess 62, a major portion of shank 65 extends beyond the lower surface of base 41.

An elongated resistance element 70, of conventional wire-wound construction and circular transverse cross section, is rigidly secured to base 41, as by means of a suitable cement, the resistance element extending along surface 50 and in engagement with the side surface 56 of rib 51. One fiat end face of the resistance element directly engages pedestal 57. The other end face of the resistance element lies in a common plane with the corresponding end face of return conductor 63. The dimensions of rib 51, return conductor 63, and resistance element 70 are such that movable contact elements, later described, which lie in a common plane parallel to surface 56 can simultaneously engage the return conductor and the resistance element, as will be clear from FIG. 8.

Resistance element 70 is equipped with two lead elements 71 and 72 which are in all respects identical with lead element 64 save that their thin tip portions 73 and 74, respectively, do not slant away from the other portions of the lead elements. While lead element 64 is welded to return conductor 63 prior to insertion of the lead element 64 into bore 59 in base 41, welding of lead elements 71 and 72 to the resistance element 70 is accomplished after these lead elements and the resistance element have been put in place. Thus, lead element 71 is inserted downwardly into bore 60, lead element 72 is inserted downwardly into bore 61, the resistance element is properly disposed on surface 50, and tip portions 73 and 74 of the lead elements are then welded to the resistance element, notches 52 and 54 affording access to the resistance element for accomplishing the welding operations.

End wall 47 of cap 42 includes a thicker portion 75, adjacent top wall 43, and a thinner portion 76, which includes the free edge of the end wall. The thicker portion 75 is provided with a circular, outwardly opening recess 77, and a cylindrical through bore 78 which is coaxial with recess 77. Adjacent end wall 46, cap 42 is provided with a flange 79 which depends from top wall 43 and is parallel with end wall 46. Flange 79 is of such length that, when cap 42 is in its finally installed position on base 41, the free end of flange 79 will be spaced slightly above the upper end of pedestal 57. A generally U-shaped notch 80 is provided in flange 79, the notch opening away from top wall 43. The bottom of notch 80 is beveled, as indicated at 81, the bevel extending for approximately half the thickness of the flange and slanting upwardly and away from end wall 46.

The potentiometer includes a metal lead screw 82 having a threaded body portion 83, an enlarged head 84 which is slotted at 85 to cooperate with a screwdriver or like turning tool, a plain right cylindrical portion 86 between head 34 and threaded body portion 83, and a tip flange 87 at the end of the lead screw opposite head 84, there being a short right cylindrical portion 88 between tip flange 87 and the threaded body of the lead screw. The lead screw is inserted into cap 42 before the cap is applied to base 41. An O-ring 89 is first placed on the lead screw in a position surrounding cylindrical portion 86. The lead screw is then inserted through bore 78 in end wall 47 until O-ring 89 is engaged between head 84 and the bottom of recess 77. As the lead screw approaches full insertion, the end thereof carrying flange 87 is pivoted away from top wall 43, such pivotal movement being allowed by reason of a slight clearance between circular portion 86 and the wall of bore 78 and a similar clearance between the periphery of circular head 84 and the side wall of the circular recess 77. With the lead screw so pivotally displaced, additional pressure is applied thereto in order to initially compress O-ring 89 and bring flange 87 to a location such that the lead screw can be pivoted toward top wall 43, with cylindrical portion 88 entering into notch 80 and the tip flange 87 engaging the side of flange 79 which is directed toward end wall 46. Such engagement between the tip flange 87 and flange 79 holds O-ring 89 under compression.

Lead screw 82 drives a movable contact unit indicated generally at 90 and shown in FIGS. 7-9. Unit 90 comprises an insulating member, indicated generally at 91, and a contact member indicated generally at 92 and formed as an integral unit from spring sheet metal.

Insulating member 91 is formed as an integral piece of suitable polymeric insulating material, such as polytetrafluoroethylene. Member 91 comprises a thin tubular body portion 93 which, in its normal relaxed or undistorted condition, is in the form of a right cylindrical tube. At the ends of body portion 93, member 91 is provided with flanges 94 and 95, respectively, these flanges each having a circular aperture of the same size as, and aligned with, the inner periphery of tubular body portion 93. Flange 94 is relatively thicker than flange 95 and has a rectangular peripheral shape corresponding to that de fined by the inner surfaces of cap 42. Thinner flange 95 is circular. Considering insulating member 91 as a whole, the member can be said to have a through bore, defined by the openings in the end flanges and the interior of tubular body portion 93. The through bore is of such diameter that the member can be slipped easily over the threaded body portion 83 of the lead screw and, advantageously, will then embrace the crest of the thread of the lead screw with a light sliding contact.

Contact member 92 includes a flat body portion 96 which is generally rectangular. A pair of contact arms 97 and 98 are bent from a common edge of the body portion 96 of the contact member and extend in generally S-like configuration to terminate in contact tips 99 and 100, respectively. Contact tips 99 and 100 are curved cylindrically, with a common axis extending parallel to the plane of body portion. 96.

The edge of body portion 96 from which the two contact arms are bent extends transversely of the elongated potentiometer when the movable contact unit is in its operative position. Two of the three remaining edges of body portion 96 extend parallel to each other and at right angles to the edge from which the contact arms are bent. From one of these two edges, there is bent upwardly from body portion 96 an arm 101. Similarly, an arm 102 is bent upwardly from the other of the two parallel edges just mentioned. Arms 101 and 102 converge away from body portion 96. Arm 101 is provided with a straight dimple 103 which is of circular transverse cross section and which slants at an angle, relative to body portion 96,

which is essentially the same as the angle of the thread of the lead screw.

With contact member 92 in its normal relaxed or undistorted condition, the space between arms 101 and 102 (taking dimple 103 into consideration) is inadequate to accommodate the thin tubular body portion 93 of insulating member 91 when body portion 93 is in undistorted condition. Due to the resilient nature of arms 101 and 102, the contact member 92 can be forced onto the body por tion of member 91, reaching the position seen in FIG. 8. With the parts in such assembled relation, the spring force provided by arms 101 and 102 urges dimple 103 against the outer surface of tubular body portion 93 and, because of the thin flexible nature of the tubular body portion 93, a small delineated area thereof is distorted inwardly by dimple 103.

Considering FIG. 7, it will be noted that the thread of lead screw 83 has a substantial pitch and that the crest of the thread is of well rounded transverse cross section. Accordingly, adjacent turns of the thread are spaced apart by a substantial distance, and the thread presents no sharp edges. Considering FIG. 9, it will be seen that the width of dimple 103 is significantly smaller than the space between adjacent turns of the lead screw thread. Accordingly, the portion of body portion 93 of member 93 which is distorted inwardly by dimple 103 extends substantially entirely into the space between the two adjacent turns of the thread, and the action of the dimple is not to force the flexible insulating material of member 91 against the crest of the thread. The action of dimple 103 in distorting the thin wall of body portion 93 inwardly is thus essentially the same as the action of dimple 27 in distorting the tubular body portion 19 of the embodiment seen in FIGS. 1-5.

From FIGS. 7 and 9, it will be seen that the width of arms 101 and 102, that is, the dimension of those arms in the direction of the axis of the lead screw in the completely assembled device, is slightly less than the space between flanges 94 and 95. Hence, the two arms engage body portion 93 over most of the length of the body portion, yet are freely accommodated between the two flanges. The overall length of body portion 93 is such that the body portion embraces more than three full turns of the thread of the lead screw. Save for the presence of dimple 103, arms 101 and 102 apply only a light inward spring pressure against the tubular body portion 93 and the body portion is in turn supported against this pressure by the rounded crest of the lead screw thread. The arrangement is such that the rounded crest of the thread slida'bly supports the tubular body portion 93, without the tubular body portion being significantly distorted, save by the dimple 103.

From FIG. 8, it will be seen that the close proximity between the inner walls of cap 42 and three of the edges of the rectangular flange 94 of insulating member 91 is such that the insulating member, and therefore the entire movable contact unit 90, can rotate about the axis of the lead screw only a minimal amount. According, whenever the lead screw is rotated in either direction, an edge of flange 94 will come into engagement with the adjacent wall of cap 42, preventing rotation of the movable contact unit, and continued rotation of the lead screw will therefore result in travel of the movable contact unit along the lead screw in a direction depending upon the direction of rotation of the lead screw.

The surface of flange 79 which is directed toward end wall 47 constitutes a stop surface to be engaged by flange 94 of insulating member 91 whenever the rotation of the lead screw in a direction causing the movable contact unit to advance toward flange 79 is continued adequately to provide the necessary travel of the movable contact unit. In the event of continued rotation of the lead screw after such engagement of the flange 94 with flange 79, the resilient nature of arms 101 and 102 allows dimple 103, and the small portion of the Wall of tubular body portion 93 which is displaced by the dimple, to ride over the crest of the lead screw thread, thus providing a clutch action which prevents such continued rotation of the lead screw from damaging the potentiometer. Similarly, the surface of the thicker portion 75 of end wall 47 which is directed toward the opposite end of the potentiometer constitutes a stop surface to be engaged by flange 95 of insulating member 91 in the event that the lead screw is rotated in a direction, and to such an extent, as will cause such engagement. In the event of such engagement, a similar clutch action occurs.

Contact arms 97 and 98 are of such dimensions that, when arms 101 and 102 are properly engaged with insulating member 91, contact tips 99 and 100 slidably engage return conductor 63 and resistance element 70, respectively, and are urged resiliently thereagainst by reason of compressive deformation of the two contact arms. Since the contact member 92 is an integral metal piece, such engagement of tips 99 and 100 with the return conductor and the resistance element is effective to establish an electrical connection between the resistance element and the return conductor at the particular point at which the movable contact unit is then positioned. Insulating member 91 acts to completely isolate the movable contact member 92 electrically from the metal lead screw and, accordingly, the only exterior electrical connections involved are those established via lead elements 64, 71 and 72.

The final positional relationship between the lead screw, the movable contact unit, the return conductor and the resistance element is established by securing cap 42 to base 61. Employing ultrasonic welding techniques, the base 41 is stationarily supported, as in a jig, the cap 42 is properly positioned on the base, and an ultrasonic tool is employed to vibrate the cap at a suitable high frequency while a downward pressure is applied to the cap. Frictional forces occurring at the free edges of walls 44-47 and the mating surface portions of the base generate heat adequate to raise the temperature of the thermoplastic material of the base and cap, in the location of the free edges just mentioned, to the fusion point and the cap is accordingly fused to the base. Since downward pressure is applied to the cap during this operation, fused material provided by the cap is caused to flow around the cross bars of lead elements 64, 71 and 72, so that the upper portions of the cross bars are, in effect, embedded in the fused polymeric material, as seen in FIGS. 12 and 13. Since the fused polymeric material at the location of each of the cross bar-accommodating recesses comes from the lower edge portion of the respective side wall 44, 45, and since, as seen in FIGS. 8 and 13, the cross bar-accommodating recesses are offset inwardly from the respective side wall so that the side wall overlaps approximately onehalf of the recess, the fused polymeric material does not completely close the top of the recess. The polymeric material does, however, completely enclose the upper edge portions of the curved ends 66a and completely fill the space between those ends. Further, the bottom surface 661), FIG. 10, of each cross bar is held firmly against the flat bottom wall of the recess, first by the downward force applied to cap 42 and then permanently by the fused polymeric material which surrounds the upper portion of the cross bar. It is to be noted that face 68 of the shank 65 of the lead element, and the corresponding face of the cross bar 66, lie in flush engagement with the inner wall of the bore 59, this being the wall spaced inwardly from the respective side wall. Accordingly, for each lead element 64, 71 and 72, the cross bar of the lead element, the bottom wall of the cross bar-accommodating recess, the inner wall of the lead element-accommodating bore, and the fused polymeric material in which the cross bar is embedded coact to provide a seal, between the lead element and the casing, which is effective to prevent foreign material from entering the casing. The ultrasonic welding operation results in final seating of the cross bars in their respective recesses, and final seating of the return conductor in groove 55. The return conductor is accordingly rigidly support inter-mediate its ends, by reason of being welded to the now-rigidly supported lead element 64, and each end portion of the return conductor is in turn supported by the corresponding bottom portion of groove 55. It is unnecessary to employ adhesive or other securing means for the return conductor.

Since the O-ring 89 is held under compression between the head of the lead screw and the bottom wall of recess 77, and since the ultrasonic welding operation employed to secure cap 42 to base 41 provides a continuous fused joint between the cap and base, the cavity defined by the housing is effectively sealed against entry of foregin material.

With cap 42 in its final position on base 41, the tip flange 87 of the lead screw is disposed immediately adjacentto the upper end face of pedestal 57. Accordingly, the pedestal is effective to assure retention of cylindrical portion 88 of the lead screw in notch In the embodiment shown in FIG. 14, the return conductor 63a is welded as hereinbefore described to the tip portion 67a of lead element 64a. One end portion of return conductor 63a is supported by engagement with the pedestal 57a and a short, grooved rib portion 510:, as hereinbefore described with reference to the construction shown in FIGS. 7-13. The other end of the return conductor 63a, however, simply rests upon the flat upper face of the second short rib portion 51b, this rib portion having no grove, and there being no other rib portion employed.

FIGS. 15-17 illustrate a movable contact unit, indicated generally at 104, constructed in accordance with another embodiment of the invention. Here, the unit again comprises an insulating member and a contact memher 106. Insulating member 105 is formed as an integral piece from polytetrafiuoroethylene or other suitable flexible polymeric material, and comprises a thin tubular body portion 107 and identical circular end flanges 108 and 109 which are annular and project outwardly from the respective ends of the tubular body portion 107.

Contact member 106 is formed as an integral piece from thin spring sheet metal of good electrical conductivity. The contact member comprises a rectangular base portion 110, contact arms 111 and 112, and two additional arms 113 and 114 which extend upwardly from base portion 110, that is, from the side thereof which is opposite contact arms 111 and 112. Arm 113 is provided with a straight elongated dimple 115 extending at an angle, relative to base portion 110, which conforms to the angle of the thread of lead screw 116.

The tubular body portion 107 of insulating member 105 is slipped over lead screw 116, so that the inner surface of the body portion lightly embraces the crest of the thread of the lead screw. Arms 113 and 114 of movable contact member 106 converge at an angle such that, with tubular body portion 107 supported on the lead screw, the tubular body portion can be accommodated between arms 113 and 114 only when those arms are distorted resiliently away from each other. Accordingly, the movable contact member 106 is applied to the insulating member by spreading arms 113 and 114 apart and forcing them over the body portion of the insulating member, until the contact member reaches the fully engaged position seen in FIG. 15. With the parts so disposed, dimple 115 is effective to distort a relatively small portion of the wall of tubular body portion 107 inwardly into the space between adjacent turns of the lead screw thread, so that the movable contact unit is operatively coupled to the lead screw.

To prevent the movable contact unit from turning with the lead screw in this embodiment of the invention, each contact arm 111, 112 is provided with a rounded portion disposed to come into direct sliding engagement with the respective adjacent side wall 117, 118 of the casing in the event that the movable contact unit commences to rotate in the corresponding direction. As seen in FIG. 15, contact arms 111, 112 have flat rectangular body portions extending from bends 119, 120, respectively, at an acute angle relative to base portion 110, as seen in FIG. 17. The free end portions of arms 111, 112 terminate respectively in integral contact tips 121, 122 which are bent cylindrically about an axis transverse to the contact arms and are concave with respect to base portion 110. Contact tips 121, 122 project outwardly beyond the outer edges of the respective contact arm body portions so that, in the assembled device, the outer ends of the contact tips are immediately adjacent to but spaced slightly from casing side walls 117, 118, respectively, when the contact arms are parallel with the side walls and tips 121 and 122 are properly engaged with return conductor 123 and resistance element 124, respectively. The entire outer end portion of each contact tip 121, 122 is bent circularly toward base portion 110, providing the rounded portions 125 and 126, respectively. Since these end portions are initially part of the right cylindrical contact tip, extending through 180, and since the entire end portion is bent circularly, the rounded portions 125 and 126 present only smooth curved surfaces to engage side walls 117, 118, and engagement of one of the rounded portions 125, 126 with the adjacent side wall will not significantly impede travel of the movable contact unit along the lead screw.

Provision of the rounded, side wall-engaging portions 125, 126 at the free ends of the contact arms is particularly advantageous because the side wall-engaging portions are disposed at the maximum practical distance from the central axis of the lead screw. Thus, even a relatively small amount of rotation of the movable contact unit provides the angular displacement of the contact tips necessary to cause the appropriate one of portions 125, 126 to come into engagement with the respective one of side walls 117, 118.

The embodiment of the invention illustrated in FIG. 18 employs a lead screw 216 with a thread of triangular cross section, so that the crest of the thread is sharp and capable of readily penetrating the wall of the insulating member 205. Arm 213 of movable contact member 206 is provided with an elongated straight dimple 215 which is parallel to the side edges of arm 213. Accordingly, when the movable contact unit 204 is assembled as shown, dimple 215 lies in a plane which extends at right angles to the longitudinal axis of the lead screw, and the portion of the wall 207 of member 205 which is deformed by the dimple therefore crosses the sharp crest of the lead screw thread twice (once for each of two adjacent turns of the thread). The force applied to wall 207 by the dimple and arm 213 is adequate to cause the crest of the lead screw thread to be embedded in the polymeric material of wall 207 at both points of crossing. In effect, the deformed portion of wall 207 assumes the configuration of the lead screw thread, so that the lead screw is operatively coupled to the insulating member 205 and therefore to the entire movable contact unit 204.

While particularly advantageous embodiments of the invention have been chosen for illustrative purposes, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims. Thus, for example, the wire wound resistance elements shown and described herein can obviously be replaced by metallic film resistance elements or cermet resistance elements, for example.

What is claimed is:

1. In a rectilinearly adjustable potentiometer of the type comprising a housing defining a cavity, an elongated resistance element mounted in the cavity, a lead screw rotatably mounted on the housing and having a threaded body extending within the cavity and parallel with the resistance element, and a return conductor mounted in the cavity parallel to the resistance element, the combination of an insulatingrnember comprising a thin flexible member embracing the threaded body of the lead screw; and a contact member carried by said insulating member and movable therewith, said contact member comprising a contact arm and a portion engaging said thin flexible member on the side thereof which faces away from the threaded body of the lead screw, said portion of said contact member deforming a small area of said thin flexible member inwardly into operative engagement with the threaded body of the lead screw such that rotation of the lead screw causes the combination of said insulating member and said contact member to travel along the lead screw, said contact arm having a contact slidably engaged with the resistance element.

2. A potentiometer according to claim 1, wherein said portion of said contact member is of such small extent that said small area of said thin flexible member is of significantly smaller extent, in a direction lengthwise of the lead screw, than the spae between adjacent turns of the thread of the lead screw.

3. A potentiometer according to claim 1, wherein said thin flexible member is tubular and said insulating member further comprises transverse annular outwardly projecting flanges located each at a different end of said thin flexible member.

4. A potentiometer according to claim 3, wherein one of said flanges has at least one edge portion lying in such immediate proximity to a wall of the housing that any substantial rotation imparted by the lead screw to the combination of said insulating member and said contact member will cause said edge portion of said one flange to engage the wall of the housing, such engagement holding the combination of said insulating member and said contact member from further rotation.

5. A potentiometer according to claim 1, wherein said contact member is an integral spring sheet metal member having an annular body embracing said thin flexible member, said portion of said contact member is an inwardly displaced part of said body.

6. A potentiometer according to claim 5, wherein said annular body of said contact member, when in relaxed and undistorted condition, has a transverse dimension significantly smaller than the corresponding transverse dimension of said thin flexible member when the latter embraces the lead screw.

7. A potentiometer according to claim 6, wherein the length of said thin flexible member is substantially greater than the pitch of the thread of the lead screw, and said portion of said contact member is approximately midway between the ends of said thin flexible member.

8. A potentiometer according to claim 1, wherein said thin flexible member is tubular, and said contact member is an integral spring sheet metal member including a body lying adjacent and parallel to said thin flexible member, and a pair of arms projecting from said body in generally the same direction, the arms of said pair converging away from said body, when in relaxed undistorted condition, to such an extent that said thin flexible member, when embracing the threaded body of the lead screw, can be accommodated between the arms of said pair only when the same are resiliently spread apart, said portion of said contact member constituting a part of one arm of said pair.

9. A potentiometer according to claim 8, wherein said insulating member is an integral member of polymeric material and further comprises two transverse annular outwardly projecting flanges each at a different end of said thin flexible member, said flanges being substantially thicker than said thin flexible portion, said pair of arms extending through the space between said flanges.

10. A potentiometer according to claim 8, wherein said body of said contact member is of rectangular plan configuration and has a first edge extending transversely of the lead screw and second and third edges extending parallel to the lead screw, said contact arm being bent away from said body along said first edge, the arms of said pair being each bent away from said body along a different one of said second and third edges.

11. A potentiometer according to claim 10 and further comprising a second contact arm bent away from said body along said first edge and having a contact slidably engaging the return conductor.

12. A potentiometer according to claim 1, wherein said contact member is provided with a rounded portion spaced laterally from the lead screw and disposed to swing into sliding engagement with a wall of the housing in event of rotation of said contact member with the lead screw.

13. A potentiometer according to claim 12, wherein said contact member comprises a second contact arm, said arms being spaced apart transversely of the housing, each of said arms having a contact tip at the free end thereof, the portion of each of said tips which is nearer the corresponding housing wall providing a rounded surface for sliding engagement with the housing wall.

14. A potentiometer according to claim 12, wherein said contact member is an integral member of spring sheet metal and includes a second contact arm provided with a rounded portion for sliding engagement with a Wall of the housing, said arms being spaced apart transversely of the housing, said arms being of substantial width and each terminating in a contact tip at the free end thereof, said contact tips extending transversely of the housing and each having an end disposed adjacent the corresponding wall of the housing, said ends of said contact tips constituting said rounded portions.

15. In a potentiometer of the type comprising a housing defining a cavity, and an elongated resistance element mounted in the cavity, the combination of a metal lead screw mounted on the housing and having a threaded body extending within the cavity and parallel to the resistance element; an insulating member having a thin flexible tubular body embracing the threaded body of the lead screw and a transverse annular outwardly projecting flange at one end of said tubular body, said flange being substantially thicker than said tubular body and having an edge portion disposed in immediate proximity to a wall portion of the housing so as to engage the wall portion to prevent rotation of said insulating member in the event said insulating member is turned as a result of rotation of the lead screw; and a contact member carried by said insulating member and slidably engaging the resistance element, said contact member deforming a portion of said thin flexible body inwardly into operative engagement with the thread of the lead screw.

16. In a potentiometer of the type comprising a housing defining a cavity, an elongated resistance element mounted in the cavity, and a lead screw mounted on the housing and having a threaded body extending within the housing and parallel to the resistance element, the combination of a tubular insulating member embracing the threaded body of the lead screw; and a contact member carried by said insulating member and having a contact slidably engaging the resistance element, said contact member having arm portions spaced apart from each other across the lead screw, each of said arm portions having an outwardly directed rounded protuberance disposed to engage a wall portion of the housing and hold said contact member against rotation.

References Cited UNITED STATES PATENTS THOMAS J. KOZMA, Primary Examiner US. Cl. X.R. 338--18l, 202 

